Double ballistic pendulum



July 25, 1961 w. BLEAKNEY ETAL 2,993,372

DOUBLE BALLISTIC PENDULUM Filed March 4, 1947 2 Sheets-Sheet 2 NVENTORS WALKER BLEAK/VEY VLADIMIR. ROJA/VSKY LOU/5' A. .DELSAS50 GEORGE 7. REYNOLDS 0% 4 b-ATTOR'ET RAYLKRAMER Patented July 25, 1961 2,993,372 DOUBLE BALLISTIC PENDULUM Walker Bleakney, Princeton, N.J., Vladimir Rojansky, Schenectady, N.Y., Louis A. Delsasso, Aberdeen, Md., George T. Reynolds, Princeton, N..J., and Ray L. Kramer, Washington, D.'C., assignors to the United States of America as represented by the Secretary of the Navy Filed Mar. 4, 1947, Ser. No. 732,266 6 Claims. (Cl. 73-467) This invention relates to an improved device for experimentally determining exterior ballistics data and a method therefor.

The invention has for an object, to provide a pendular device for testing the behavior of projectiles in flight and that of armor plate through which the projectile is fired.

Another object of this invention is to provide a pair of pendulurns aligned in such manner that a projectile fired through an armor plate secured to the first pendulum, may emerge therefrom and impinge on the second pendulum in order to measure the velocity of the projectile as it enters and leaves the plate.

It is a further object of the invention to provide a mounting for both pendulurns which will provide stability and yet allow the pendulurns to be placed close together.

It is a further object of the invention to provide styli for recording pendulum deflections, which styli are positioned in the same vertical plane as the centers of gravity of the pendulums.

It is a still further object of the invention to provide a novel structure to accomplish the above stated functions.

Other objects and advantages of the invention will be apparent during the course of the following description.

In the drawings, forming a part of this specification and in which like numerals are employed to designate like parts throughout the same,

FIG. 1 is a perspective view of the pendulurns and their supporting tapes,

FIG. 2 is a plan view of the pendulurns showing their relative positions,

FIG. 3 is a side elevation of the novel structure, and

FIG. 4 is a detail view of one of the post brackets to which the supporting tapes are attached.

In the drawings, 1 represents a T shaped support having a test armor plate 2 secured thereto, and having three suspension posts 3 secured thereto at spaced points. A block 4 is disposed directly behind test plate '2 and is provided with similar suspension posts 5. The upper ends of all of posts 3 and 5 are disposed in a common horizontal plane and the support 1 and block 4 are counterweighted in such manner that their centers of gravity lie in the aforesaid horizontal plane along a line normal to test plate 2.

Each of posts 3 and 5 is suspended from a horizontal planar support which may conveniently be a roof or ceiling (not shown), by vertically extending tapes 7 to form with the T shaped support 1 and the block 4, a pair of ballistic pendulurns. Guy wires or tapes 8 are secured to at least two of the posts on each of these pendnlums and diverge upwardly to points of support on the ceiling surface; in this manner the guy tapes 8 tend to restrict lateral or rotary movement of the pendulums.

The test plate support 1 is provided with a stylus 9 (FIG. 3) adapted to cooperate with a fixed recording plate 10 to indicate the maximum movement of the test plate and its support. The block 4 is provided with a similar stylus 11 which cooperates with a fixed recording plate 12 to indicate the maximum movement of block 4.

Both of these styli are supported by arms 9' and 11' respectively, which arms are pi votally supported in the vertical plane containing the centers of gravity of the two pendulurns formed by the test plate support 1 and block 4. Stylus '9 is placed as close to the center of gravity of pendulum 1 as possible.

Springs 13 and 14 are provided to urge the styli 9 and 11 into engagement with their respective recording plates 10 and 12.

A series of counterweights indicated generally at 15, are provided to move the centers of gravity of pendulurns 1 and 4 vertically into the plane of the supports 5, and laterally into the line of fire.

A blast shield 16, rigidly supported in any desired manner and provided with an aperture just large enough to clear the projectile, is disposed immediately in trout of the test plate 2 in order to minimize the effects of blast from the gun on the pendulum 1, 2.

In use, a bullet or projectile is fired along a line of fire passing through the centers of gravity of both pendulurns and the movement of both of the pendulurns is recorded by means of styli 9, 11 and plates 10 and 12.

Since the first pendulum 1 is moved in an arcuate path, due to its supporting tapes 7 and 8, during the interval between the entrance of the bullet into the test plate 2 and its emergence therefrom, it is desirable to place the block 4 close to the maximum position of deflection of the test plate in order that the bullet may enter the block '4 as near to the center of gravity thereof as possible. The use of a T shaped support 1 for test plate 2 allows close spacing of the two pendulurns without sacrifice of stability. 7

Prior devices for measuring the velocity of the projectile when it struck the armor plate, required elaborate measuring devices interposed between the gun and the armor plate for a considerable distance. An appreciable error was thus introduced due to the fact that the velocity of the projectile was attenuated rapidly over this distance, due to air friction. In the present invention, the gun may be placed very close to the armor and this source of error is thus eliminated.

The positions of styli 9 and 11 in the vertical plane containing the line of fire and the centers of gravity of the pendulurns tends to minimize errors in recording movement of the pendulum due to any slight rotary movements of the pendulums.

For the purpose of gaining insight into the method of the double pendulum, it is instructive to treat each pendulum as an ideal pendulum. Let us adopt the following notation:

M mass of bullet v striking velocity of bullet v residual velocity of bullet B, momentum of blast M mass of plate M mass of transmission pendulum M mass of terminal pendulum and bullet R length of suspension of transmission pendulum R length of suspension of terminal pendulum v velocity of transmission pendulum immediately after impact v velocity of terminal pendulum immediately after impact s horizontal displacement of transmission pendulum s horizontal displacement of terminal pendulum P, impulse given to transmission pendulum by bullet E energy of translation of transmission pendulum E energy of rotation of transmission pendulum E energy absorbed by tar-get plate E striking energy of bullet E residual energy of bullet g, acceleration due to gravity The condition for conservation of angular momentum about the upper point of support of the transmission pendulurn-considered as an ideal pendulumis b s+ 1 l'i' p) 1 1+ b r 1+ T( 1 1) Here I is the moment of inertia of thefiupporting tape and is given by IT=MTR12/3 where M is the mass of the supporting tapes. Thus Eq. 1 becomes b s+ 1+ ..+MT 1+ b r Similar considerations for the terminal pendulum show that M V =M 2V2 and, therefore, that r= Z 2 b Thus, from Eqs. 2 and 3,

5 1'i' pi T/ )1+ 2 2 (3a) If h is the height of rise of the transmission pendulum, then, from the principle otconservation of energy, 1+ p+ T 1 1+ p+ T) 1 1-i- T) 1\/ 1 1 or, to a good approximation, noting that s is much smaller than R and that M +M +M 3 is approximately equal to M +M /2M We have One thus obtains for the residual and striking velocities of the bullet the expressions,

The values of R and R are determined from the periods T by the usual relation for the ideal pendulum,

The impulse P given to the transmission pendulum by the'bullet is In practice this energyamounts to about 1 ft. lb. and should be taken into account for pendulums Whose mass amounts to 100 lb. or so.

(ii) The energy of rotation E of the transmission pendulum is negligible, for the shots are directedat the center of massof the pendulum and hence there should be no energy expended upon rotation. Theresults of 4 experiments shows that the energy of rotation is about 0.001 ft. lb. and may be neglected entirely in comparison with the energy of plastic deformation.

(iii) The energy absorbed by the target plate, E is the essential quantity in armor work. Of this total absorbed energy, that part due to vibration depends largely on the dimensions of the plate. In field use the armor closely approximates an infinite plate; this approximation is also valid for large plates shot in testing. For small plates the deviation from the infinite case must be considered in the interpretation of the data. In some practical cases the energy'of vibration of an infinite plate may amount to as much as 5 percent of the total energy absorbed by the plate. A 4 x 4 x Ai-in. plate, simply supported around the edges and shot in the center With a .30-cal. projectile, deviates vfrom the infinite case by about 2 percent.

It is seen from the foregoing considerations that the relation between the energies involved is the bullet, and E is neglected as previously explained. The equation may be solved for E thus,

where v and v are given by Eqs. 6 and 7, and B is given by Eq. 9. 7

It is obvious that other modes of applying the principles of this invention may be employed.

We claim:

1. In a device for determining exterior ballistic data of projectiles and of test pieces into which the projectiles are fired; a first T-shaped pendulum comprising a movable support and means for securing atest piece thereto, a second pendulum disposed to oscillate out of contact with said first pendulum but adjacent thereto, supporting means for both of said pendulums and for constraining said pendulums for movementgenerally along a predetermined line coincident with the flight path of a projectile to be fired through the test piece and into the second pendulum, the center of gravity of each of said pendulums being disposed along said line, and recording means operatively connected to each pendulum to record the maximum movement of each pendulum, said recordingmeans including scribing devices pivotally secured to each of said pendulums and extending in a vertical plane containing the centers of gravity of each of said pendulums'and into contact with stationary recording means, said second pendulum at all times during the determination remaining out of contact with and oscillating independently of said first pendulum and the test piece carried thereby.

2. In a device for determining exterior ballistic data of projectiles and of test pieces into which the projectiles are fired, a first pendulum comprising a support and means for securing a test piece thereto, a second pendulum disposed to oscillate out of contact With said first pendulum and in close spaced adjacency thereto, supporting means for both of said pendulums for constraining said pendulums for movement generally along a predetermined line, said supporting means including a plurality of flexible members secured at their lower ends adjacent laterally spaced extremities of said first and second pendulums and secured at their upper ends along lines substantially parallel to said predetermined line to a support having a horizontal planarsurface, means to minimize undesired lateral and rotary movements of said pendulums, said last means including guy elements respectively having a turnbuckle secured at the lower end thereof, said guy elements .beingifastenedto said pendulurns and diverging upwardly :and: secured to points of support on said horizontal surface, said last means further including a plurality of suspension posts having the upper ends thereof disposed in a common horizontal plane, each of said suspension posts including an upstanding member having at least one laterally projecting member secured thereto and at the upper end portion thereof, said laterally projecting member being provided with a recess for receiving substantially one-half of said turnbuckle, a securing plate provided with a recess complementary to the recess in the laterally projecting member for receiving substantially the other half of said turnbuckle, and means for detachably fastening the securing plate to the laterally projecting member, recording means operatively connected with each pendulum to record the maximum movement of each pendulum, the center of gravity of each of said pendulums being disposed along said predetermined line, and said predetermined line being coincident with the path of flight of a projectile to be fired through the test piece of said first pendulum and into said second pendulum in order to determine the energy required to puncture the test piece and the velocity of the bullet emerging therefrom, said second pendulum at all times during the determination remaining out of contact with and oscillating independently of said first pendulum and the test piece carried thereby.

3. In a device for determining exterior ballistic data of projectiles and test pieces into which the projectiles are fired, a first T-shaped pendulum comprising a sup port and means for securing a test piece thereto, a second pendulum disposed adjacent said first pendulum, supporting means including suspension posts for both of said pendulums adapted to constrain said pendulums for movement generally along a given line, said support comprising a transverse portion for rigidly supporting a test piece and a second portion extending substantially perpendicularly to said transverse portion to a position abreast of said second pendulum, the supporting means for said first pendulum being secured to remote portions of said support, the center of gravity of each of said pendulums being disposed along said line, and means associated with each pendulum to record the maximum movement of each of said pendulums when a projectile is fired along said line through the test piece and into said second pendulum in order to determine the energy required to puncture the test piece and the velocity of the projectile emerging therefrom, said pendulums being supported to oscillate independently one another and disengage from one another during all phases of the determination, mov able counter weights on said pendulums for altering the location of the center of gravity thereof to a position of coplanar relationship with the horizontal plane in which the upper ends of the suspension posts lie and laterally to a colinear position with the line of fire.

4. A double ballistic pendulum comprising two gravity suspended bobs, a first including a T-shaped support having means for securing a test piece to the T-leg, and the second having straight side members, supporting means for each bob disposed to retain the second bob on a level with and partially within space defined by the intersecting T-members of the first bob whereby the centers of gravity of the first bob is in line with and adjacent to that of the second while enabling said first bob to oscillate free from contact with the second.

5. A device for determining the energy required by the projectile in flight along a predetermined line of fire to perforate a given test piece and for determining the projectile velocities before and after perforation comprising a first pendulum including a movable support and means for securing a test piece thereto, a second pendulum disposed to oscillate out of contact with said first pendulum but adjacent thereto, adjustable counterweights on each of said pendulums, supporting means including supporting posts for both pendulums enabling their oscillatory movement in arcuate paths in the same plane, the center of gravity of both pendulums being adjustable by movement of said counterweights to a position along said predetermined line of fire and to a position in a horizontal plane within which the upper ends of said supporting posts lie, movement measuring means for each pendulum, the supporting means suspending the pendulums to enable contact free oscillation of the first pendulum from that of the second when the test piece is struck and perforated by a projectile and the second pendulum is penetrated at its center of gravity by the perforating projectile.

6. A device for determining ballistic characteristics of projectiles and armor plate comprising a first pendulum having means for securing a specimen of armor plate thereto, a second pendulum having a mass of substantially imperforate material carried thereby, suspension means including suspension posts on both pendulums for suspending the plate and mass in the path of flight of a projectile fired through the plate and into the mass, adjustable counterweights carried by each pendulum for altering the location of the center of gravity of each pendulum in a vertical direction to a position with the horizontal plane in which the upper ends of said suspension posts lie and in a lateral direction to a position colinear with the line of fire of the projectile, said first pendulum and plate remaining out of contact with said second pendulum and mass during all phases of the determination, said pendulums being supported for independent oscillation during all phases of the determination and in close spaced adjacency at the time the projectile is fired through the plate, and means on each pendulum for recording the oscillation thereof in response to a sequential transfer of energy from the projectile to the plate and mass.

References Cited in the file of this patent UNITED STATES PATENTS 1,709,638 Thwing Apr. 16, 1929 2,319,342 Perry May 18, 1943 2,476,297 Harris July '19, 1949 FOREIGN PATENTS 248,888 Great Britain Mar. 18, 1926 396,057 Great Britain July 26, 1933 OTHER REFERENCES Physics for Colleges, by Sheldon, Kent, Paton and Miller, published September 1926, pages 90, 91 and 92. 

